Up to the present time, production of a fluorescent lamp has generally been carried out by dissolving nitrocellulose as a binder in an organic solvent such as butyl acetate, etc. to prepare a solution, in which a phosphor is then suspended, adding an adhesive material consisting of a divalent metal borate having a composition of BaO.CaO.B.sub.2 O.sub.3 for the purpose of tenacious adhesion of the fluorescent film to the inner surface of a glass tube to the suspension, followed by adequately mixing, coating the resulting phosphor-coating liquid onto the inner surface of the glass tube, drying, baking to decompose and remove the binder and thus forming the fluorescent film.
Herein, the method of using such an organic coating composition has various problems of difficulty in handling, safety, poisonous character, economy, etc. and lately, has gradually been replaced by a method of using an aqueous phosphor-coating liquid. When using the above described adhesive material for the organic coating composition, i.e. BaO.CaO.B.sub.2 O.sub.3 in an aqueous phosphor-coating liquid, however, there arises a problem that BaO.CaO.B.sub.2 O.sub.3 tends to be dissolved in water as a solvent and its precipitate hinders the decomposition of the binder during baking, thus retaining carbon in the fluorescent film.
Thus, there have been proposed methods of using, as an adhesive material for an aqueous phosphor-coating liquid, aluminum oxide (Japanese Patent Publication No. 28668/1965), calcium pyrophosphate (Japanese Patent Publication No. 7440/1970), a low melting point glass (Japanese Patent Publication No. 1778/1991), etc.
When using aluminum oxide or calcium pyrophosphate as an adhesive material, however, there arises a problem that since a fluorescent film and a glass tube are bonded through van der Waals force, the bonding strength is weak and in particular, a circular fluorescent lamp obtained by bending the tube after forming the fluorescent film tends to often meet with peeling of the fluorescent film. In the case of using a low melting point glass as an adhesive material on the other hand, a fluorescent film is bonded to a glass tube by fusion and the bonding strength is sufficient, but the water resisting property is inferior.
Thus, it has been proposed to render a low melting point glass as an adhesive material less water-soluble by adding at least one of Al.sub.2 O.sub.3, SiO.sub.2 and Sb.sub.2 O.sub.3 and an oxide of at least one of Y and lanthanoids (Ln) to a borate of at least one divalent metal selected from Mg, Ca, Sr, Ba and Zn (Japanese Patent Publication No. 1778/1991).
In this method, however, there arises a problem that the water resisting property is not sufficient and when the adhesive material is crushed in fine grains and suspended in water, the fine grains are dissolved in water. Accordingly, it has eagerly been desired to develop an adhesive material such that when a phosphor-coating liquid is coated onto a glass tube, the bonding strength of the fluorescent film with the glass tube is sufficently large and decomposition of the binder is not hindered.
The present invention aims at providing a glass composition for bonding a phosphor (which will hereinafter be referred to as "glass composition"), which has a sufficient bonding strength for bonding the fluorescent film with the glass tube, does not hinder decomposition of the binder and is effective as an adhesive material, and to a fluorescent lamp using the glass composition.